Power over Ethernet (PoE), which is outlined in IEEE Std 802.3-2005 clause 33 (the PoE standard), refers to a technique for delivering power and data to an electronic device via Ethernet cabling. PoE eliminates the need for a separate power source to deliver power to attached powered devices. In a PoE system, a power sourcing equipment (PSE) device provides a power supply to electronic devices, which may be referred to as powered devices (PDs), via an Ethernet cable. Such PDs may include voice over Internet protocol (VoIP) telephones, wireless routers, security devices, network devices, field devices to monitor process control parameters, data processors, other devices, or any combination thereof.
The PoE standard specifies that the PSE device perform a powered device detection operation to determine whether the PD is attached before supplying power via the Ethernet cable. To perform the PD detection operation, the PSE device applies a DC voltage (within a range of 2.8 to 10 Volts DC) to pairs of wires of the Ethernet cable and monitors a received current (Amps) or a received voltage (V) to detect a resistance (i.e., a PD detection signature) within an expected range (e.g., between 19 and 26.5 K-ohms). If the PSE device does not detect a valid resistance, the PSE device does not apply power to the Ethernet port assigned to the PD.
Once a PD has been detected, the PoE standard specifies that the PSE device may optionally perform a PD power classification operation to determine power requirements of the detected PD. Such power requirements may be used by the PSE device to manage its power budget. The PoE Standard specifies five device classes, classes zero through four (0-4), which classes define expected power consumption levels for particular categories (classes) of PDs. If the PSE device supports power classification, the PSE device applies a PD classification voltage (DC) to the Ethernet port associated with the detected PD and monitors the Ethernet port to detect a PD classification signature associated with the detected PD. A current level associated with the PD classification signature corresponds to a particular classification and specifies the PD's power classification. For example, a current signature between zero and four mA corresponds to class 0, which is also the default class for PDs that do not support classification. A current draw of 26 to 30 mA and of 36 to 44 mA corresponds to class 3 and class 4 devices, respectively. The PoE standard specifies that PDs in classes 0, 3, or 4 expect a power supply of approximately 15.4 watts. A current draw of between 9 and 12 mA corresponds to a class 1 device, and the PoE standard specifies that the PSE provide a power supply of up to approximately 4 watts to a class 1 device. A class 2 device corresponds to a current draw of 17 to 20 mA and requires the PSE to provide a power supply of approximately 7 watts of power.
A revised PoE standard, which can be referred to as the PoE “Plus” Standard (the PoE+ Standard), makes use of the class 4 classification to detect a PoE+ enabled PD. In a particular example, a current draw of 36 to 44 mA (class 4) can identify the PD as being a PoE+ enabled PD. In response to detecting a class 4 PD classification signature, under the PoE+ Standard, the PSE device may perform a PoE+ classification process, which includes one or more additional PD classification voltages applied for periods of time that are separated by classification mark voltages. During application of the PD classification voltages, the PD is expected to provide a PD classification signature (such as a current signature that corresponds to the power classification of the PD). During application of the classification mark voltages, the PD provides a PD classification mark signature (such as a classification mark current within a current range from 0.25 mA to 4 mA).
Hence, there is a need for circuitry to provide the PD detection and PD classification signatures. Embodiments disclosed herein can provide solutions to these and other problems, and offer other advantages over the prior art.